Method of reducing plasma stabilization time in a cyclic deposition process

ABSTRACT

The present invention relates to an enhanced cyclic deposition process suitable for deposition of barrier layers, adhesion layers, seed layers, low dielectric constant (low-k) films, high dielectric constant (high-k) films, and other conductive, semi-conductive, and non-conductive films. The deposition enhancement is derived from ions generated in a plasma. The techniques described reduce the time required for plasma stabilization, thereby reducing deposition time and improving efficiency.

TECHNICAL FIELD

The present invention relates generally to the field of thin filmdeposition methods commonly used in the semiconductor, data storage,flat panel display, as well as allied or other industries. Moreparticularly, the present invention relates to cyclic depositiontechniques and apparatus suitable for deposition of barrier layers,adhesion layers, seed layers, low dielectric constant (low-k) films,high-dielectric constant (high-k) films, and other conductive,semi-conductive, and non-conductive thin films.

BACKGROUND

As integrated circuit (IC) dimensions shrink and the aspect ratios ofthe resulting features increase, the ability to deposit conformal,ultra-thin films on the sides and bottoms of high aspect ratio trenchesand vias becomes increasingly important. These conformal, ultra-thinfilms may be barriers, liners, or seeds.

In addition, decreasing device dimensions and increasing devicedensities has necessitated the transition from traditional chemicalvapor deposition (CVD) tungsten plug and aluminum interconnecttechnology to copper interconnect technology. This transition is drivenby both the increasing impact of the RC interconnect delay on devicespeed and by the electromigration limitations of aluminum basedconductors for sub 0.2 micron (μm) device generations. Copper ispreferred due to its lower resistivity and higher (greater than 10times) electromigration resistance as compared to aluminum. A single ordual damascene copper metallization scheme is used since it eliminatesthe need for copper etching and reduces the number of integration stepsrequired. However, the burden now shifts to the metal deposition step(s)as the copper must fill predefined high aspect ratio trenches and/orvias in the dielectric.

Two major challenges exist for copper wiring technology: the barrier andseed layers. Copper can diffuse readily into silicon and mostdielectrics. This diffusion may lead to electrical leakage between metalwires and poor device performance. An encapsulating barrier layer isneeded to isolate the copper from the surrounding material (e.g.,dielectric or silicon (Si)), thus preventing copper diffusion into orreaction with the underlying material. In addition, the barrier layeralso serves as the adhesion or glue layer between the patterneddielectric trench or via and the copper used to fill it. The dielectricmaterial can be a low dielectric constant, i.e., low-k material whichtypically suffers from poorer adhesion characteristics and lower thermalstability than traditional oxide insulators. Consequently, this placesmore stringent requirements on the barrier material and depositionmethod. An inferior adhesion layer will, for example, lead todelamination at either the barrier-to-dielectric or barrier-to-copperinterfaces during any subsequent anneal or chemical mechanicalplanarization (CMP) processing steps leading to degradation in deviceperformance and reliability. Ideally, the barrier layer should be thin,conformal, defect free, and of low resistivity.

In addition, electroplating fill requires a copper seed layer, whichserves to both carry the plating current and act as the nucleationlayer. The seed layer should be smooth, continuous, of high purity, andhave good step coverage with low overhang. A discontinuity in the seedlayer will lead to sidewall voiding, while gross overhang will lead topinch-off and the formation of top voids.

Both the barrier and seed layers which are critical to successfulimplementation of copper interconnects require the deposition of highpurity, conformal, ultra-thin films at low substrate temperatures.

Physical vapor deposition (PVD) or sputtering has been adopted as amethod of choice for depositing conductor films used in ICmanufacturing. This choice has been primarily driven by the low cost,simple sputtering approach whereby relatively pure elemental or compoundmaterials can be deposited at relatively low substrate temperatures. Forexample, refractory based metals and metal compounds such as tantalum(Ta), tantalum nitride (TaN_(x)), other tantalum containing compounds,tungsten (W), tungsten nitride (WN_(x)), and other tungsten containingcompounds which are used as barrier/adhesion layers can be sputterdeposited with the substrate at or near room temperature. However, asdevice geometries have decreased, the step coverage limitations of PVDhave increasingly become an issue since it is inherently a line-of-sightprocess. This limits the total number of atoms or molecules which can bedelivered into the patterned trench or via. As a result, PVD is unableto deposit thin continuous films of adequate thickness, control, andcoverage to coat the sides and bottoms of high aspect ratio trenches andvias with the necessary degree of conformality. Moreover,medium/high-density plasma and ionized PVD sources developed to addressthe more aggressive device structures are still not adequate and are nowof such complexity that cost and reliability have become seriousconcerns.

CVD processes offer improved step coverage since CVD processes can betailored to provide conformal films. Conformality ensures the depositedfilms match the shape of the underlying substrate, and the filmthickness inside the feature is uniform and equivalent to the thicknessoutside the feature. CVD requires comparatively high depositiontemperatures, suffers from high impurity concentrations, which impactfilm integrity, and have higher cost-of-ownership due to long nucleationtimes and poor precursor gas utilization efficiency. Following thetantalum containing barrier example, CVD Ta and TaN films requiresubstrate temperatures ranging from 500° C. to over 800° C. and sufferfrom impurity concentrations (typically of carbon and oxygen) rangingfrom several to tens of atomic % concentration. This generally leads tohigh film resistivities (up to several orders of magnitude higher thanPVD) and other degradation in film performance. These depositiontemperatures and impurity concentrations make CVD Ta and TaN unusablefor IC manufacturing, in particular for copper metallization and low-kintegration.

A plasma-assisted (PACVD) or plasma-enhanced (PECVD) CVD approach hasbeen demonstrated using tantalum pentabromide (TaBr₅) as the precursorgas to reduce the deposition temperature. Ta and TaN_(x) films weredeposited from 350° C. to 450° C. and contained 2.5 to 3 atomic %concentration of bromine.

Atomic layer chemical vapor deposition (ALCVD) or atomic layerdeposition (ALD) has been proposed as an alternative method to CVD fordepositing conformal, ultra-thin films at comparatively lowertemperatures. ALD is similar to CVD except that the substrate issequentially exposed to one reactant at a time. Conceptually, it is asimple process: a first reactant is introduced onto a heated substratewhereby it forms a monolayer on the surface of the substrate. Excessreactant is pumped out. Next a second reactant is introduced and reactswith the first reactant to form a monolayer of the desired film via aself-limiting surface reaction. The process is self-limiting since thedeposition reaction halts once the initially adsorbed (physi- orchemisorbed) monolayer of the first reactant has fully reacted with thesecond reactant. Finally, the excess second reactant is evacuated. Theabove sequence of events comprises one deposition cycle. The desiredfilm thickness is obtained by repeating the deposition cycle therequired number of times.

In practice, ALD is complicated by the painstaking selection of aprocess temperature setpoint wherein both: 1) at least one of thereactants sufficiently adsorbs to a monolayer and 2) the surfacedeposition reaction can occur with adequate growth rate and film purity.If the substrate temperature needed for the deposition reaction is toohigh, desorption or decomposition of the first adsorbed reactant occurs,thereby eliminating the layer-by-layer process. If the temperature istoo low, the deposition reaction may be incomplete (i.e., very slow),not occur at all, or lead to poor film quality (e.g., high resistivityand/or high impurity content). Since the ALD process is entirelythermal, selection of available precursors (i.e., reactants) that fitthe temperature window becomes difficult and sometimes unattainable.

Continuing with the TaN example, ALD of TaN films is confined to anarrow temperature window of 400° C. to 500° C., generally occurs with amaximum deposition rate of 0.2 Å/cycle, and can contain up to severalatomic percent of impurities including chlorine and oxygen. Chlorine isa corrosive, can attack copper, and lead to reliability concerns.

In conventional ALD of metal films, gaseous hydrogen (H₂) or elementalzinc (Zn) is often used as the second reactant. These reactants arechosen since they act as a reducing agent to bring the metal atomcontained in the first reactant to the desired oxidation state in orderto deposit the end film. Gaseous, diatomic hydrogen (H₂) is aninefficient reducing agent due to its chemical stability, and elementalzinc has low volatility and is generally incompatible with ICmanufacturing. Due to the temperature conflicts that plague the ALDmethod and lack of kinetically favorable second reactant, seriouscompromises in process performance result.

In order to address the limitations of traditional thermal or pyrolyticALD, radical enhanced atomic layer deposition (REALD) or plasma-enhancedatomic layer deposition has been proposed whereby a downstreamradio-frequency (RF) glow discharge is used to dissociate the secondreactant to form more reactive radical species which drives the reactionat lower substrate temperatures. Using such a technique, Ta ALD filmshave been deposited at 0.16 to 0.5 Å/cycle at 25° C., and up toapproximately 1.67 Å/cycle at 250° C. to 450° C. Although REALD resultsin a lower operating substrate temperature than all the aforementionedtechniques, the process suffers from several drawbacks. Highertemperatures must still be used to generate appreciable depositionrates. Such temperatures are still too high for some films ofsignificant interest in IC manufacturing such as polymer-based low-kdielectrics that are stable up to temperatures of only 200° C. or less.REALD remains a thermal or pyrolytic process similar to ALD and even CVDsince the substrate temperature provides the required activation energyfor the process and is therefore the primary control means for drivingthe deposition reaction.

In addition, Ta films deposited using REALD contain chlorine as well asoxygen impurities, and are of low density. A low density or porous filmleads to a poor barrier against copper diffusion since copper atoms andions have more pathways to traverse the barrier material. Moreover, aporous or under-dense film has lower chemical stability and can reactundesirably with overlying or underlying films, or with exposure togases commonly used in IC manufacturing processes.

Another limitation of REALD is that the radical generation and deliveryis inefficient and undesirable. RF plasma generation of radicals used asthe second reactant such as atomic H is not as efficient as microwaveplasma due to the enhanced efficiency of microwave energy transfer toelectrons used to sustain and dissociate reactants introduced in theplasma. Furthermore, having a downstream configuration whereby theradical generating plasma is contained in a separate vessel, locatedremotely from the main chamber where the substrate is situated,negatively impacts the efficiency of transport of the second radicalreactant.

SUMMARY

In one aspect, the invention features a method of depositing a film ontoa substrate in a chamber. The method comprises (a) introducing at leastone precursor vapor into the chamber to adsorb at least one layer of theprecursor vapor on the substrate; (b) operating an excitation energysource to generate a plasma; (c) reacting the adsorbed layer of theprecursor vapor with ions and radicals to form the film; and (d) outsideof step (b), operating the excitation energy source at a power levelless than at step (b) such that an afterglow or a plasma is maintained.

Various implementations of the invention may include one or more of thefollowing features. The plasma is generated by an excitation energysource selected from the group consisting of microwave power, DC power,RF power, ultraviolet light, x-rays, a high DC field, a molecular beam,an ion beam, and combinations thereof.

In yet another aspect, the invention is directed to a sequential methodfor depositing a film onto a substrate in a deposition chamber. Themethod comprises (a) introducing a reactant gas into the chamber toadsorb at least one layer of the reactant gas onto the substrate; (b)removing any excess reactant gas from the chamber; (c) introducing atleast one ion generating feed gas into the chamber; (d) introducing atleast one radical generating feed gas into the chamber; (e) operating anexcitation energy source at a first power level to generate a plasmafrom the ion generating feed gas and the radical generating feed gas toform ions and radicals; (f) prior or subsequent to step (e), operatingthe excitation energy source at a second power level greater than zerobut less than the first power level; (g) exposing the substrate to theions and the radicals; (h) modulating the ions; and (i) reacting theadsorbed layer of the reactant gas with the ions and the radicals todeposit the film.

Various implementations of the invention may include one or more of thefollowing features. In the above-described method, step (b) isaccomplished by evacuating or purging the chamber. The method isrepeated until the film achieves a desired thickness. The method furtherincludes exposing the substrate to at least one additional reactant gas.The substrate is simultaneously exposed to the ions and the radicals.The substrate is exposed to the ions after exposure to the radicals. Atstep (f), the excitation source is operated to form an afterglow or aplasma.

In still another aspect, the invention features a method for depositinga film onto a substrate in a chamber. The method comprises (a)introducing at least one reactant gas into the chamber to adsorb atleast one layer of the reactant gas on the substrate; (b) generating aplasma from an ion generating feed gas to form ions and a radicalgenerating feed gas to form radicals, by operation of an excitationenergy source; (c) electrically biasing the substrate to a negativepotential; (d) exposing the substrate to the ions and the radicals; (e)modulating the ions; (f) reacting the adsorbed layer of the reactant gaswith the ions and the radicals to deposit the film; and (g) prior orsubsequent to the generating step, operating the excitation energysource at a power level greater than zero but less than its level ofoperation at the generating step.

Other implementations of the invention may include one or more of thefollowing features. The method is repeated until the film achieves adesired thickness. At step (g), the excitation energy source is operatedto form an afterglow or a plasma. The reactant gas is exposed to anafterglow to form a more reactive reactant gas.

In another aspect, the invention is directed to a method of depositing afilm onto a substrate wherein at least one precursor vapor is introducedinto a deposition chamber to adsorb at least one layer of the precursorvapor on the substrate. An excitation energy source is operated at afirst power level to generate an ion-generating plasma. The adsorbedlayer of the precursor gas is reacted with ions and radicals to form afilm. The first three steps of the above-described method and repeated.Between the first three steps of the above-described method and thefourth step of the above-described method, the excitation energy sourceis operated at a second power level greater than zero but less than thefirst power level.

The invention can include one or more of the following advantages. Itcan increase throughput by reducing the time required for plasmaformation. The invention provides costs savings. Less power is requiredto run the deposition system and component life is improved.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a deposition system suitable for modulatedion-induced atomic layer deposition (MII-ALD).

FIG. 2A depicts a timing sequence for an MII-ALD method incorporatingperiodic exposure of the substrate to ions.

FIG. 2B is another timing sequence for an MII-ALD method incorporatingperiodic exposure of the substrate to ions.

FIG. 3A shows the MII-ALD method utilizing ion flux modulation to varythe substrate exposure to ions.

FIG. 3B shows the timing of the MII-ALD method utilizing ion energymodulation to vary the substrate exposure to ions by varying thesubstrate bias.

FIGS. 4A, 4B, 4C, 4D, 4E and 4F show methods of modulating the MII-ALDprocess.

FIGS. 5 a and 5 b show an electrostatic chuck (ESC) system suitable formodulating the ion energy in the MII-ALD process: a) in topologicalform; and, b) as an equivalent electrical circuit.

FIG. 6 shows a timing sequence for an improved sequential method ofdepositing a film.

FIG. 7 shows a timing sequence for an improved continuous method ofdepositing a film.

FIG. 8 shows a timing sequence for another method of depositing a filmwherein an excitation energy source is operated between the filmformation steps.

DETAILED DESCRIPTION

The present invention relates to methods and apparatus useable for thedeposition of thin films of one or more elements at low temperature. Thepresent invention relates to cyclical deposition techniques andapparatus suitable for the deposition of barrier layers, adhesionlayers, seed layers, low dielectric constant (low-k) films, highdielectric constant (high-k) films, and other conductive,semi-conductive, and nonconductive thin films.

FIG. 1 illustrates a deposition system suitable for modulatedion-induced atomic layer deposition (MII-ALD). The MII-ALD systemdescribed herein incorporates a means of modulating the exposure of thesubstrate to ions. By modulating (1) the ion flux; (2) the energy of theions striking the substrate; or a combination of (1) and (2), thedeposition reaction can be precisely toggled “on” or “off”. If the ionflux or energy is at a “low” state, then no deposition results ordeposition occurs so slowly that essentially no deposition results. Ifthe impinging ion flux or energy is at a “high” state, then depositionoccurs. Since a substrate (which may be a “bare” substrate, e.g., asilicon wafer before any films have been deposited, or it may be asubstrate which may already have had one or more films deposited on itssurface) is maintained at a low substrate temperature, the first andsecond reactants do not thermally react with any appreciable rate or donot react at all. Instead, the deposition reaction only takes place wheneither the ion flux or ion energy is toggled to a suitable “high state”.The desired film thickness is built up by repeating the ion pulses(either of flux or energy) the required number of cycles. An MII-ALDsystem and method are described in U.S. Pat. No. 6,416,822, entitled“Continuous method for depositing a film by modulated ion-induced atomiclayer deposition (MII-ALD)”, and U.S. Pat. No. 6,428,859, entitled“Sequential Method for depositing a film by modulated ion-induced atomiclayer deposition (MII-ALD)”, both of which are assigned to the assigneeof the subject application and which are both incorporated herein byreference.

In the deposition system of FIG. 1, all of the ion/radical generatingfeed gases and the precursor gases are introduced into a main bodychamber 190 via a distribution showerhead 171 comprised of a series ofarrays or apertures 175. However, other means for uniformly distributinggases essentially parallel or perpendicular to a face of a substrate 181may also be used. It will be appreciated that although the showerhead171 is shown to be above the substrate 181 to direct a gas flowdownwards towards the substrate 181, alternative lateral gasintroduction schemes are possible with this embodiment. Various lateralgas introduction schemes are described in U.S. Publication No.:US2002/0197402A1, Publication Date: Dec. 26, 2002, entitled “System fordepositing a film by modulated ion-induced atomic layer deposition(MII-ALD)”, application Ser. No. 10/215,711, filed Aug. 8, 2002, whichis herein incorporated by reference.

In the embodiment of FIG. 1, a source of RF bias power 160 is coupled toone or more ESC electrodes 603 in a substrate pedestal 182, whichincludes insulation 183, via an impedance matching device 150. The ESCelectrodes 603 may be of any arbitrary shape. The RF bias power providespower for both ion generation during modulated ion induced atomic layerdeposition and energy control of the generated ions. The applied RF biaspower is used to generate a plasma 172 in a main process chamber 180,for example, between the substrate 181 and the showerhead 171 todissociate feed gases 110, 130 to generate ions 177 and radicals 176 andto induce a negative potential V_(bias) 185 (i.e., a DC offset voltagetypically −10V to −80V at ≦475 W RF power and 0.1-5 Torr pressure) onthe substrate 181. The negative potential V_(bias) 185 modulates theenergy of the positively charged ions in the plasma and attracts thepositively charged ions toward the surface of the substrate. Thepositively charged ions impinge on the substrate 181, driving thedeposition reaction and improving the density of the deposited film. Theion energy is more specifically given by E=e|V_(p)|+e|V_(bias)|, whereV_(p) is the plasma potential (typically 10V to 20V) and V_(bias) is thenegative potential V_(bias) 185 induced on the substrate 181. Thenegative potential V_(bias) 185 is controlled by the applied RF biaspower. For a given process region geometry, the induced negativepotential V_(bias) 185 increases with increasing RF bias power anddecreases with decreasing RF bias power.

Controlling the RF bias power also controls the density and hence thenumber of ions generated in the plasma. Increasing the RF bias powergenerally increases the ion density, leading to an increase in the fluxof ions impinging on the substrate. Higher RF bias powers are alsorequired for larger substrate diameters. A preferred power density is≦0.5 W/cm², which equates to approximately ≦150 W for a 200 mm diametersubstrate. Power densities ≧3 W/cm² (greater than about 1000 W for a 200mm diameter substrate) may lead to undesired sputtering of the depositedfilm.

The frequency of the RF bias power can be 400 kHz, 13.56 MHz, or higher(e.g. 60 MHz, etc.). A low frequency (e.g. 400 kHz), however, can leadto a broad ion energy distribution with high energy tails which maycause excessive sputtering. The higher frequencies (e.g., 13.56 MHz orgreater) lead to tighter ion energy distributions with lower mean ionenergies, which is favorable for modulated ion-induced ALD depositionprocesses. The more uniform ion energy distribution occurs because theRF bias polarity switches before ions can impinge on the substrate, suchthat the ions see a time-averaged potential.

As shown in FIG. 1, a source of applied DC bias can also be coupled tothe ESC substrate pedestal 182. The source can be a DC power supply 510coupled by a center tap 518 to a voltage source 525 with the ability tovary the voltage or exhibit an infinite impedance. Optionally, avariable impedance device 605 may be coupled in series between thevoltage source 525 and the center tap 518 of the DC power supply 510.The voltage source 525 is itself coupled to a waveform generator 535.The waveform generator may be a variable-type waveform generator. Anexemplary variable-type waveform generator may be controlled by acontrol computer 195 and have a variable waveform at different timeswithin a given process and may additionally have a non-periodic outputsignal. The source of applied DC bias can be coupled to the ESCsubstrate pedestal 182 by RF blocking capacitors 601 that both provide aDC open for the DC power supply 510 and prevent RF energy fromcorrupting the DC power supply 510.

In MII-ALD, the same plasma is used to generate both ions 177 (used todrive the surface reactions) and radicals 176 (used as the secondreactant). The MII-ALD system utilizes ion imparted kinetic energytransfer rather than thermal energy (e.g., REALD, ALD, PECVD, CVD, etc.)to drive the deposition reaction. Since temperature can be used as asecondary control variable, with this enhancement films can be depositedusing MII-ALD at arbitrarily low substrate temperatures (generally lessthan 350° C.). In particular, films can be deposited at or near roomtemperature (i.e., 25° C.) or below.

The system of FIG. 1 contains a substantially enclosed chamber 170located in substantial communication with or substantially within a mainchamber body 190. The feed gases 110, 130 are delivered to the plasmasource chamber 170 via valving 115 and 116, and a gas feed line 132.Typical feed gases 130 used for ion generation include, but are notrestricted to, Ar, Kr, Ne, He, and Xe. Typical feed gases 110 (e.g.,precursor B) used for radical generation include, but are not restrictedto H₂, O₂, N₂, NH₃, and H₂O vapor. The ions 177 are used to deliver theenergy needed to drive surface reactions between the first adsorbedreactant and the generated radicals 176.

A first reactant 100 (e.g., precursor A) is introduced to the chamber170 via valving 105 and the gas feed line 132. Precursor A may be anyone or more of a series of gaseous compounds used for depositingsemiconductors, insulators, metals or the like that are well-known inthe art (e.g, PDEAT (pentakis(diethylamido)tantalum), PEMAT(pentakis(ethylmethylamido)tantalum), TaBr₅, TaCl₅, TBTDET (t-butyliminotris(diethylamino)tantalum), TiCl₄, TDMAT(tetrakis(dimethylamido)titanium), TDEAT(tetrakis(diethylamino)titanium), CuCl, Cupraselect®((Trimethylvinylsilyl) hexafluoroacetylacetonato Copper I), W(CO)₆, WF₆,etc.).

FIG. 2A depicts a sequence for an MII-ALD method incorporating periodicexposure of the substrate to ions. In this method, ion exposure 230begins with the introduction of the second precursor 220 (especiallywhen plasma generated radicals 176 are used as the second precursor orreactant). This figure illustrates one embodiment of MII-ALD utilizingthe apparatus described in FIG. 1. This results in a sequential ALDprocess as follows:

-   -   1) First exposure 200: The substrate 181 is exposed to a first        gaseous reactant 100 (e.g., precursor A), allowing a monolayer        of the reactant to form on the surface. The substrate 181 may be        at a temperature above or below the decomposition temperature of        the first gaseous reactant although it is preferable for the        temperature to generally be less than approximately 350° C.    -   2) First removal 210: The excess reactant 100 is removed by        evacuating 214 the chamber 180 with a vacuum pump 184. Note that        prior to the first exposure 200, the chamber was initially        evacuated 212. Alternatively, the chamber may be purged rather        than evacuated.    -   3) Second exposure 220: Unlike conventional ALD, the substrate        181 is simultaneously exposed to ions 177 and a second gaseous        reactant during this step with the substrate 181 (e.g., wafer)        biased to a negative potential V_(bias) 185. The ions will        strike the wafer 181 with an energy (E) approximately equal to        (e|V_(bias)|+e|V_(p)|) where V_(p) is the plasma 172 potential        (typically 10V to 20V) and V_(bias) is the negative potential        bias 185 induced on the substrate. With the activation energy        now primarily supplied by ions 177 instead of thermal energy,        the first and second (chemi- or physi-sorbed) reactants react        via an ion-induced surface reaction to produce a solid thin        monolayer of the desired film at a reduced substrate temperature        below conventional ALD. The deposition reaction between the        first and second reactants is self-limiting in that the reaction        between them terminates after the initial monolayer of the first        reactant 100 is consumed.    -   4) Second removal 210: The excess second reactant is removed by        again evacuating or purging 216 the chamber 180.    -   5) Repeat: The desired film thickness is built up by repeating        the entire process cycle (steps 1-4) many times.

Additional precursor gases (e.g., 120, 140) may be introduced andevacuated as required for a given process to create tailored films ofvarying compositions or materials. As an example, an optional exposuremay occur in the case of a compound barrier of varying composition. Forexample, a TaN_(x)/Ta film stack is of interest in copper technologysince TaN_(x) prevents fluorine attack from the underlying fluorinatedlow-k dielectrics, whereas the Ta promotes better adhesion andcrystallographic orientation for the overlying copper seed layer. TheTaN_(x) film may be deposited using a tantalum containing precursor(e.g., TaCl₅, PEMAT, PDEAT, TBTDET) as the first reactant 100 (precursorA) and a mixture of atomic hydrogen and atomic nitrogen (i.e. flowing amixture of H₂ and N₂ into the plasma source 172) as the second reactantto produce a TaN_(x) film. Simultaneous ion exposure is used to drivethe deposition reaction. Next a Ta film may be deposited in a similarfashion by using atomic hydrogen (as opposed to a mixture of atomichydrogen and nitrogen) as the second reactant. An example of a tailoredfilm stack of differing materials can be the subsequent deposition of acopper layer over the TaN_(x)/Ta bi-layer via the use of a coppercontaining organometallic (e.g., 25 Cu(TMVS)(hfac) or(Trimethylvinylsilyl) hexafluoroacetylacetonato Copper I, also known bythe trade name CupraSelect®, available from Schumacher, a unit of AirProducts and Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, Calif.92009) or inorganic precursor (e.g. CuCl) shown as precursor C 120 inFIG. 1. The copper layer can serve as the seed layer for subsequentelectroless or electroplating deposition.

A variant of the method shown in FIG. 2A is illustrated in FIG. 2B whereion exposure is initiated after the second reactant exposure. FIG. 2Bdepicts a sequence for a method incorporating periodic exposure of thesubstrate 181 to ions 177. In this variant of the method, ion exposure280 begins with the removal 250 of the second precursor 260 (especiallywhen the second precursor or reactant is not subjected to a plasma).Typically, this is the case where the second precursor or reactant isnot a plasma-generated radical.

In another embodiment of the MII-ALD process, a substrate 181 heated(e.g., to a low temperature of less than or equal to 350° C.) orunheated is simultaneously exposed to a first reactant and a secondreactant, and subjected to modulated ion 177 exposure. By modulating (1)the ion flux (i.e. the number of ions hitting the substrate per unitarea per unit time); (2) the energy of the ions striking the substrate;or a combination of (1) and (2), the deposition reaction can beprecisely toggled “on” or “off”. Since the substrate 181 is maintainedat a low substrate temperature, the first and second reactants do notthermally react with any appreciable rate or do not react at all whenthe ion flux or energy is toggled to a “low” state. Instead, thedeposition reaction only takes place when either the ion flux or ionenergy is toggled to a suitable “high state”. Ion flux or energymodulation can vary generally from 0.1 Hz to 20 MHz, preferably from0.01 KHz to 10 KHz. During deposition, the main process chamber 180pressure can be maintained in the range of generally 10² to 10⁻⁷ torr,more preferably from 10¹ to 10⁻⁴ torr, depending on the chemistryinvolved. The desired film thickness is attained via exposure of thesubstrate to the suitable number of modulated ion flux or energy pulsecycles. This MII-ALD scheme results in a continuous deposition process.The modulation can be either of the ion flux via the plasma power or ofthe ion energy via an applied periodic wafer bias.

The MII-ALD method utilizing ion flux modulation to control thedeposition cycle is illustrated conceptually in FIG. 3A, with the fluxmodulation scheme described more explicitly in FIGS. 4A and 4C. FIG. 3Adepicts the MII-ALD method utilizing ion flux modulation 320 to vary thesubstrate 181 exposure to ions 177. Note that the second reactant 310,e.g., radicals, is synchronized with the ion flux via 320 plasma powermodulation, causing a periodic exposure of the substrate to ions andradicals. Varying the power 160 delivered to the plasma 172 can vary theion flux from little or none to maximum ion production. Plasma powermodulation can take the form of variations in frequency (periodicity),magnitude, and duty-cycle. Increasing plasma power 160 leads toincreasing plasma 172, and hence, increased ion 177 density. Since thedeposition process is ion-induced, having little or no ion bombardmentwill essentially stop the deposition process, whereas increased ionbombardment will cause deposition to occur. A constant wafer bias 185(DC in FIG. 4C or RF in FIG. 4A) is applied to define the ion energy ofthe modulated ion flux in this embodiment and is chosen to besufficiently high so that ion-induced surface reactions can occur. Notethat in this embodiment since the plasma power 160 is used to generateboth ions 177 and radicals 176, the second reactant (e.g., radicals)flux 310 is synchronized with the ion flux 320 pulses. The radical feedgas 110 (H₂ for example) flow, however, does not change. Instead, theradical flux 310 (e.g., fraction of H₂ which is converted to atomic H)is modulated.

Alternatively, subjecting the substrate 181 to a non-constant wafervoltage bias 185 can vary the incoming ion energy at a fixed plasmapower 160 (i.e., ion flux). This embodiment of MII-ALD is illustratedconceptually in FIGS. 3B, and more explicitly in FIGS. 4B and 4D. FIG.3B shows the MII-ALD method utilizing ion energy modulation 350 to varythe substrate 181 exposure to ions 177 by varying the substrate bias185. The applied bias 185 can take the form of variations in frequency(periodicity), magnitude, and duty-cycle. A DC, as shown in FIG. 4D, orRF (e.g., 400 kHz, 2 MHz, 13.56 MHz, etc.), as shown in FIG. 4B, powersupply can be used. When the wafer potential is “low” (e.g., near or atzero with respect to ground), the incoming ions 177 do not have enoughenergy to induce surface deposition reactions. When the wafer 181potential is “high” (e.g., at a significant negative potential relativeto ground), the incoming ions 177 will have the necessary energy toinduce surface deposition reactions via collision cascades. In such afashion, the deposition can be turned “on” or “off” by modulating thewafer bias voltage 185, and hence the impinging ion 177 energy. Typicalwafer voltages can range from generally −20 V to −1000 V, but preferablyin the −25 V to −500 V range, and more preferably in the −50 V to −350 Vrange during deposition. The bias voltage 185 is coupled to the wafervia the pedestal 182. The substrate pedestal 182 may be an electrostaticchuck (ESC) to provide efficient coupling of bias voltage to thesubstrate. The ESC is situated in the main processing chamber 180 andcan be cooled via a fluid coolant (preferably a liquid coolant) and/orheated (e.g., resistively) to manipulate the substrate temperature.

As illustrated in FIGS. 5 a and 5 b for the case of an applied DC bias,the electrostatic chuck is a ESC 500 (bulk resistivity generally greaterthan 10¹³ ohm-cm) rather than one whose bulk material effects aredominated by the Johnson-Rahbek (JR) effect (bulk resistivity between10⁸ and 10¹² ohm-cm). Typically, the substrate potential is a complexfunction of the voltage of the electrostatic “chucking” electrodes ifthese voltages are established relative to a reference potential, but issimplified in the case of (non-JR) ESC. However, if a power supply 510that powers the ESC 500 is truly floating, i.e., the entire system has ahigh impedance to the chamber 180 potential (usually ground) includingthe means of supplying power, then the substrate potential can bearbitrary. In particular, if the ESC power supply 510 is alsocenter-tapped 518, then the wafer potential can be established byconnecting the center tap 518 to the output of a power amplifier 520. Awaveform generator 535 coupled to the power amplifier 520 can becontrolled by a control computer 195 (FIGS. 1 and 6) to, for example,periodically drop the substrate potential to a negative value for acertain period of time or apply a given frequency to the ESC 500. It isdesired to have independent control of the magnitude, frequency(periodicity), and duty cycle of this substrate bias pulse train. Suchan ESC system is depicted in FIG. 5, which shows an ESC system suitablefor modulating the ion energy in the MII-ALD process: a) in topologicalform; and, b) as an equivalent electrical circuit.

The deposition rate is affected by the choice of the critical bias pulsetrain variables: the magnitude, frequency (periodicity), and duty cycle.Preferably, when the bias frequency is high (e.g., 100 Hz-10 KHz) with ashort duty cycle (e.g., less than 30%), reducing the net, time-averagedcurrent (which can cause substrate potential drift, de-chuckingproblems, or charge-induced device damage) while providing a chargerelaxation period wherein the ion charges accumulated during ionexposure can redistribute and neutralize.

Once the deposition rate is calibrated for a particular recipe(Angstroms/cycle), the ability to accurately determine the filmthickness by counting cycles is a further benefit of this modulationscheme. The higher the frequency, the finer the resolution of thiscritical deposition process performance metric.

Alternatively, the substrate potential can be modulated by imparting aninduced DC bias to the substrate by applying RF power to the pedestal.Preferably, the RF power is coupled into the ESC electrodes. FIGS. 4A-4Fillustrate the methods of modulating the MII-ALD process. In FIG. 4A, anRF bias power B₂ is applied to the substrate pedestal 182 imparting aninduced DC bias V₂ to the substrate while the plasma (either microwaveor RF) power 400 is varied periodically between a high P₁ and a low P₂power state. In FIG. 4B, plasma (either microwave or RF) power 410 isconstant P₁ while an RF bias power, applied to the substrate pedestal182, is varied between a low B₁ and a high B₂ bias state (V₁ and V₂ arethe DC offset or bias voltages resulting from the applied RF biaspower). In FIG. 4C, a negative DC bias 425 is applied to the substratepedestal 182 while the plasma (either microwave or RF) power 420 isvaried periodically between a high P₁ and a low power P₂ state. In FIG.4D, plasma (either microwave or RF) power is constant 430 while a DCbias 435 applied to the substrate pedestal 182 is varied between a zeroV₁ and a negative voltage state V₂. In FIG. 4E, a mechanical shutterperiodically occludes the ion source. All the while, the plasma power440 (either microwave or RF) and substrate voltage 445 are heldconstant. In FIG. 4F, a source area that is smaller than the substrate181 is preferably used. In this case, plasma (either microwave or RF)power 450 is constant, a negative DC substrate bias 455 is constant, andthe source and substrate 181 are moved relative to each other 457,exposing only a portion of the substrate 181 at a time.

This process utilizes independent control over the three constituents ofplasma—ions, atoms, and precursors. Decoupling these constituents offerimproved control over the deposition process.

An added benefit of using MII-ALD is that with proper choice of thesecond reactant, selective ion-enhanced etching and removal of unwantedimpurities can be performed. As an example, for many chemistries, thepreferred second reactant is monatomic hydrogen (H) 176. Simultaneousenergetic ion and reactive atomic H bombardment will cause selectiveremoval of unwanted impurities (e.g., containing carbon, oxygen,fluorine, or chlorine) commonly associated with organometallicprecursors (e.g., TBTDET, PEMAT, PDEAT, TDMAT, TDEAT), and proceed withremoval rates superior to either chemical reaction (e.g., atomic H only)or physical sputtering (e.g., Ar ion only) alone. Impurities lead tohigh film resistivities, low film density, poor adhesion, and otherdeleterious film effects. Alternatively, in addition to atomic hydrogen,other reactive groups such as nitrogen atoms (N), oxygen atoms (O), OHmolecules, or NH molecules, or a combination thereof may be employed.

Another embodiment of the above-described methods is shown in FIG. 6.Here, a first precursor is exposed to a plasma 630 during itsintroduction into the chamber 180. Specifically, during the firstprecursor exposure 600 and the precursor removal 610, the excitationsource is on at some level (step 632) less than the level during thesecond precursor exposure 620 (step 634). However, at step 632, theexcitation source power should not be so high as to cause sputtering ofthe first precursor or damage to the substrate.

More specifically, as shown in FIG. 6, the plasma exposure 630 beginswith the introduction of the first precursor 600 and continues throughthe first precursor removal 614. This results in an improved sequentialALD process as follows.

-   -   1) First exposure 600: The substrate 181 is exposed to a gaseous        reactant 100 (e.g., precursor A), allowing a layer of the        reactant to form on the surface. The substrate 181, as        discussed, may be at a temperature above or below the        decomposition temperature of the first gaseous reactant. This        step may, in one embodiment, last about one-half of a second.    -   2) First removal 610: The excess reactant 100 is removed by        evacuating 614 the chamber 180 with a vacuum pump 184.        Alternatively, in another configuration, the excess reactant is        purged from the chamber. This step may also last about one-half        of a second. During the first precursor 600 and the removal 614,        the excitation source 630 is on to expose the first precursor to        a plasma. The plasma may be an Ar plasma or any combination of        plasma gases. The power 160 (RF), in one embodiment, may be        approximately 50 W during step 632.    -   3) Second exposure 620: The substrate 181 is simultaneously        exposed to ions 177 and a second gaseous reactant during this        step with the substrate 181 biased to a negative potential        V_(bias) 185. As discussed, RF power 160 may be supplied into        the plasma chamber 170 to generate both the ions 177 (e.g.,        argon-ion (Ar⁺)) and radicals 176 (e.g., H atoms). The power 160        (RF) may be approximately 425 W. This step 634 may last        approximately two seconds.    -   4) Second removal 610: The power 160 is off (plasma shutdown),        and the excess second reactant and plasma are removed by        evacuating or purging 616 the chamber 180. This step may also        last about one-half of a second. Alternatively, this step may be        shorter or longer than removal 614.

5) Repeat: The desired film thickness is built up by repeating theentire process cycle (steps 1-4) many times.

In another embodiment, the plasma exposure 630 may be discontinuedduring the removal 614. It would then be restarted during the secondexposure 620 to form an ion-generating plasma. In another embodiment,the plasma exposure may be or may not be discontinued in step 616.

The sequential method shown in FIG. 2B, where ion exposure is initiatedafter the second reactant exposure, may also be enhanced by exposing thefirst precursor to a plasma, that may or may not cause a visible glow,during the first precursor exposure and removal steps.

Similarly, the continuous processes shown in FIGS. 3A and 3B may beenhanced by the above-described plasma exposure. For instance, as shownin FIG. 7, the first precursor 700 may be subjected to plasma exposure720 (step 722) before full power is applied (step 724) during the secondprecursor 710 exposure. In one embodiment, the excitation source mayform a plasma at step 722 and an ion-generating plasma at step 724.

The method of the present invention increases the chemical reactivity ofa precursor by removing or otherwise altering at least one ligand of aprecursor molecule. The size of the precursor molecule may be changed.For instance, the size may be reduced. The polarization of the precursormolecule may also be changed. For example, the polarization may beincreased or decreased, or the sign of polarization may be changed.

As discussed above, the substrate is exposed to a sequence of discreetstates. Each of the states represent a step in a deposition cycle. Thesequence of steps is repeated to produce a film of a desired thickness.In at least one of the steps, the substrate is exposed to a vaporcontaining at least one precursor chemical. The precursor vapor adsorbsonto the substrate. The rate of adsorption is not infinite and thedensity of the adsorbed molecules is also not infinite. However, thesetwo factors influence the cycle time which impacts throughput as well asthe deposition rate per cycle. The rate of adsorption as well as thenumber of available adsorption sites is a function of the substrate andprecursor pair. Therefore, altering the precursor results in analteration of the adsorption rate and number of sites. An alterationthat cleaves part of at least one ligand, for example, exposes achemically reactive bond that in turn can be subject to its ownadsorption mechanisms with the substrate independent of the rest of theprecursor molecule. Moreover, if this alteration reduces the size of theprecursor molecule, then the Steric hindrance phenomena is minimized.Steric hindrance is the phenomena wherein a collection of moleculespreferentially space themselves out. In other words, the molecules donot like to be crowded.

The precursor gas, in one implementation, may be exposed to the plasmain the region between the gas line 132 and the showerhead 171. Theprecursor gas may also be exposed to the plasma in the region below theshowerhead.

A precursor molecule, in any event, is altered at or near the substraterather than in a remote fashion. The present invention thus provides amore reactive precursor molecule, but only in the region of interest inthe vicinity of the substrate to prevent stray or parasitic reactionswith the vapor delivery system. This sort of precracking duringnucleation also reduces nucleation delay for all substrate materials.

Multiple excitation sources may also be used with the present invention.The excitation source employed at step 632 may also be a separate sourcefrom that employed at step 634. Additionally, the excitation energyapplied at step 632 may vary from one cycle to another cycle tocompensate for effects that may vary with time or cycle numbers. Forexample, as film builds up in the chamber, it alters the ability of thechamber to transmit RF energy. This alteration can be compensated for byvarying the applied RF coupling.

Also, in other implementations, the precracked precursor exposure mayfollow some other reactive gas/plasma exposure step. Also, the plasma,in another embodiment, may be an ion-generating plasma at steps 632 and722. Moreover, the precursor may be exposed to an excited species atsteps 632 and 722. An excited species may be an excited gas which formsan ion-generating plasma, a plasma or afterglow, and/or includes ions,free electrons, or meta-stable gas atoms. Further, in another variant ofthese techniques, the power level at steps 632 and 722, for instance,may be the same as or greater than the power level at step 634 and 724.

In another embodiment, as shown in FIG. 8, the excitation energy ismaintained at some low value greater than zero during the periodsbetween plasma exposure for reacting a first precursor film and a secondprecursor. In the MII-ALD sequential and continuous processes, forexample, the substrate is periodically exposed to an ion-generatingplasma. A substantial amount of the cycle time is dedicated to theion-generating plasma step. The ion-generating plasma step, in theseprocesses, can be reduced if the non-productive time associated with theplasma stabilization time is reduced or eliminated. The method of thepresent invention does just that.

It is generally assumed that the excitation energy must be reduced tozero between the ion-generating plasma steps to prevent gas phasedecomposition of the vapors in the ambient and possible resultingcontamination of the deposited film. It has been found, however, thatthis is not the case. The present invention increases throughput byreducing or eliminating the non-productive time in a cyclic depositionprocess that occurs with each deposition cycle.

As shown in FIG. 8, the excitation source 830 may be on at some lowvalue (step 832) prior to film formation (step 834). The excitationenergy source is, thus, operated in an idling condition prior to filmformation. In the MII-ALD sequential process, for example, ions andradicals are generated at step 834, which takes place during the secondprecursor exposure 820. The idling step 832 may occur, as shown, duringthe first precursor exposure 800 and removal 810 (step 814). By idlingthe excitation energy source at step 832, the plasma is, in effect,seeded. This eliminates the incubation time or ignition delay in theplasma/ion generation step 834. In one embodiment, the power 160 (RF)may be approximately 50 W and 425 W, respectively, at steps 832 and 834.

At step 832, an afterglow rather than a plasma may be formed. Theexcitation source may also be operated at this same low value onlyduring removal 814, after the first precursor exposure 800. Theexcitation source may also be operated at this low value during removal816.

The plasma may be generated by microwave or RF power. The plasma mayalso be generated by DC power. The excitation source, in otherembodiments, may be ultraviolet light, x-rays, a high DC field, amolecular beam, an ion beam, or some other form of electromagneticradiation. The excitation source may also be some combination of thesesources.

The present invention is not limited to use with an MII-ALD system andprocess. The present invention may be used with various cyclicdeposition systems and techniques. For example, it may be used with ALD,ALCVD, pulsed nucleation layer (PNL), or pulsed deposition layer (PDL)systems and methods.

The method of the present invention can be used to deposit variousmaterials, including dielectric, oxide, semiconducting, or metal films,used in the semiconductor, data storage, flat panel display, and alliedas well as other industries. In particular, the method and apparatus issuitable for the deposition of barrier layers, adhesion layers, seedlayers, low dielectric constant (low-k) films, and high dielectricconstant (high-k) films. The deposited layer may be one or moremonolayers, or it may be a fractional layer, which is less than onemonolayer.

From the description of the preferred embodiments of the process andapparatus set forth above, it is apparent to one of ordinary skill inthe art that variations and additions to the embodiments can be madewithout departing from the principles of the present invention. As anexample, chlorine, bromine, fluorine, oxygen, nitrogen, hydrogen, otherreactants and/or radicals containing the aforementioned elements or acombination thereof, in conjunction with energetic ion bombardment, canbe used to effect etching or material removal as opposed to deposition.This is of particular importance in the cleaning of native oxides ofcopper, aluminum, silicon, and other common conductor and semiconductormaterials used in IC manufacturing. Either the deposition or etching canbe accomplished globally (as illustrated in the preceding embodiments)or may be chosen to be local to a controlled area (i.e., site-specificusing a small, ion beam point or broad-beam source scanned or otherwisestepped across the substrate, exposing only a fraction of the substratearea at any given time).

1. A cyclic method of depositing a film onto a substrate in a chambercomprising: (a) introducing at least one precursor vapor into thechamber to adsorb at least one layer of the precursor vapor on thesubstrate; (b) operating an excitation energy source at a first powerlevel to generate a plasma to form ions and radicals; (c) reacting theadsorbed layer of the precursor vapor with the ions and radicalsgenerated with the excitation energy source at the first power level toform the film; and (d) outside of steps (b) and (c), operating theexcitation energy source at a second power level, the second power levelbeing less than the first power level such that an afterglow ismaintained, and such that a plasma stabilization time for the plasmagenerated at the first power level is minimized.
 2. The method of claim1, wherein the excitation energy source is selected from the groupconsisting of microwave power, DC power, RF power, ultraviolet light,x-rays, a high DC field, a molecular beam, an ion beam, and combinationsthereof.
 3. The method of claim 1, wherein the cyclic method is repeateduntil the film achieves a desired thickness.
 4. The method of claim 1,further comprising: before step (b), removing excess precursor vaporfrom the chamber.
 5. The method of claim 1, wherein the excitationenergy source is RF power, and wherein the first power level isapproximately 425 watts and the second power level is approximately 50watts.
 6. The method of claim 1, further comprising: exposing theprecursor vapor to the afterglow or the plasma to form a more reactiveprecursor vapor.
 7. A cyclic sequential method for depositing a filmonto a substrate in a chamber comprising: (a) introducing a reactant gasinto the chamber to adsorb at least one layer of the reactant gas ontothe substrate; (b) removing excess reactant gas from the chamber; (c)introducing at least one ion generating feed gas into the chamber; (d)introducing at least one radical generating feed gas into the chamber;(e) operating an excitation energy source at a first power level togenerate a plasma from the ion generating feed gas and the radicalgenerating feed gas to form ions and radicals; (f) exposing thesubstrate to the ions and the radicals; (g) modulating the ions and theradicals; (h) reacting the adsorbed layer of the reactant gas with theions and the radicals generated with the excitation energy source at thefirst power level to deposit the film; and (i) prior or subsequent tostep (e), operating the excitation energy source at a second power levelgreater than zero but less than the first power level such that anafterglow is maintained, and such that a plasma stabilization time forthe plasma generated at the first power level is minimized.
 8. Themethod of claim 7, wherein step (b) is accomplished by evacuating thechamber.
 9. The method of claim 7, wherein step (b) is accomplished bypurging the chamber.
 10. The method of claim 7, wherein the cyclicmethod is repeated until the film achieves a desired thickness.
 11. Themethod of claim 7, further comprising exposing the substrate to at leastone additional reactant gas.
 12. The method of claim 7, wherein thesubstrate is simultaneously exposed to the ions and the radicals. 13.The method of claim 7, wherein the substrate is exposed to the ionsafter exposure to the radicals.
 14. The method of claim 7, wherein theexcitation energy source is selected from the group consisting ofmicrowave power, DC power, RF power, ultraviolet light, x-rays, a highDC field, a molecular beam, an ion beam, and combinations thereof.
 15. Acyclic method for depositing a film onto a substrate in a chambercomprising: (a) introducing at least one reactant gas into the chamberto adsorb at least one layer of the reactant gas on the substrate; (b)generating a plasma, from an ion generating feed gas to form ions and aradical generating feed gas to form radicals, by operation of anexcitation energy source at a first power level; (c) electricallybiasing the substrate to a negative potential; (d) exposing thesubstrate to the ions and the radicals generated with the excitationenergy source at the first power level; (e) modulating the ions and theradicals; (f) reacting the adsorbed layer of the reactant gas with theions and radicals to deposit the film; and (g) prior or subsequent tostep (b), operating the excitation energy source at a second power levelgreater than zero but less than the first power level such that anafterglow is maintained, and such that a plasma stabilization time forthe plasma generated at the first power level is minimized.
 16. Themethod of claim 15, wherein the cyclic method is repeated until the filmachieves a desired thickness.
 17. The method of claim 15, furtherincluding exposing the reactant gas to the afterglow to form a morereactive reactant gas.
 18. The method of claim 15, wherein theexcitation energy source is selected from the group consisting ofmicrowave power, DC power, RF power, ultraviolet light, x-rays, a highDC field, a molecular beam, an ion beam, and combinations thereof.
 19. Amethod of depositing a film onto a substrate in a deposition chambercomprising: (a) introducing at least one precursor vapor into thedeposition chamber to adsorb at least one layer of the precursor vaporon the substrate; (b) operating an excitation energy source at a firstpower level to generate a plasma that includes ions and radicals; (c)reacting the adsorbed layer of the precursor gas with ions and radicalsto form the film; (d) in a subsequent deposition cycle, repeating steps(a), (b) and (c); and (e) between the first deposition cycle and thesubsequent deposition cycle, operating the excitation energy source at asecond power level greater than zero but less than the first power levelsuch that an afterglow is maintained, and such that a plasmastabilization time for the plasma generated at the first power level isminimized.
 20. The method of claim 19, wherein the excitation energysource is selected from the group consisting of microwave power, DCpower, RF power, ultraviolet light, x-rays, a high DC field, a molecularbeam, an ion beam, and combinations thereof.